1. FIELD OF THE INVENTION
This invention relates to the hydrogenation of phenol and more particularly to the promotion of the hydrogenation of phenol to cyclohexanone in the presence of a non-promoted palladium catalyst.
2. DESCRIPTION OF THE PRIOR ART
In the hydrogenation of phenol employing a palladium catalyst, the activity of the catalyst, and hence the rate of hydrogenation, decreases with continued use of the catalyst due to impurities present in the hydrogenation reaction mixture which poison the catalyst. While processes, such as those disclosed in in U.S. Pat. Nos. 3,692,845 and 3,187,050 have been developed to purify organic compounds such as phenol to be hydrogenated, poisoning of metallic catalysts has not been entirely eliminated in large scale commercial processes due to long-term accumulation of impurities, such as those impurities which are introduced with the starting material and the hydrogen gas, and those impurities which result from apparatus corrosion. In addition, by-products formed during the reduction cycle may have an adverse effect upon the metallic catalysts. Moreover, while the process of U.S. Pat. No. 3,692,845 effectively removes carbonyl-bearing impurities from phenol by treatment of the phenol with a polyamine, any polyamine remaining in the phenol fed to hydrogenation acts as a poison to the catalyst.
To avoid the economically prohibitive alternatives of discarding poisoned catalyst or continuing to use the poisoned catalyst at a reduced rate of hydrogenation, it is desirable to promote the rate of hydrogenation, thereby overcoming the disadvantages of continued use of such poisoned palladium catalysts. The hydrogenation of phenol to cyclohexanone has been promoted by the use of "promoted palladium-on-carbon catalysts", i.e. catalysts which have been treated, prior to their addition to the hydrogenation reaction mixture, to incorporate on the catalysts a material which enhances their activity. Thus, in U.S. Pat. No. 3,076,810, cyclohexanone is produced by the hydrogenation of phenol in the presence of a sodium-promoted catalyst, i.e. a palladium catalyst which has been chemically modified, prior to its introduction to the reaction mixture, to incorporate sodium thereon. While such catalysts effect an increase in the rate of hydrogenation, the use of promoted catalysts is disadvantageous due to the expense of time and process equipment needed to treat the catalyst to incorporate the selected material thereon prior to the addition of the promoted catalyst to the hydrogenation reaction mixture. Moreover, the addition to the hydrogenation reaction mixture of compounds such as the inorganic alkaline reacting compounds of U.S. Pat. No. 3,076,810 to further enhance the hydrogenation of phenol in the presence of a promoted palladium catalyst has not avoided the expensive and time consuming procedures necessary in the preparation of the promoted catalysts themselves.